It is common, after placing a pipeline, a submarine cable, or other elongated member, at the bottom of a body of water, to bury or entrench the elongated member to avoid the effects of, for example, ocean or other water currents, passing vessels, and other hazards of the sea. Various techniques for entrenching an elongated member have been suggested and used. By far the most common is the "jetting" method by which fluids are propelled at high velocities against the subsea surface to dislodge and otherwise remove debris and other ground materials from beneath the elongated member, so that the member will fall into the resulting trench and thereby be safely situated beneath the water bottom.
Despite its versatility, as compared to other methods of trenching thus far used, the "jetting" method nevertheless has its shortcomings. For example, the jetting method has a high operating cost and a slow rate of progress, is uneconomical for deeper waters, and is relatively inefficient in certain kinds of soil composition.
As the demand for oil and gas resources has increased, those exploring offshore for these natural resources have ventured into deeper waters and more severe environments. In order to extend the trenching capability to these deeper waters, considerable effort has been devoted to the development of other trenching techniques. As a result, equipment utilizing mechanical cutters has been discussed in the literature, and various versions of such equipment have been developed, all using the same basic trenching technique. They differ from each other primarily only in certain relatively minor details. Generally, these devices are also relatively inflexible and cannot easily be adapted to different trenching applications.
It is therefore an object of the invention to provide an apparatus and method for trenching the bottom of a body of water which operates at depths up to and exceeding 3000 feet, which has a relatively low operating cost, and which has a relatively high rate of progress.
Further objects of this invention are to provide a novel cutting assembly which is capable of varying the width of the cutting swath, which flexibly varies the depth and angle of the cutting assembly, and which provides mechanical cutting means to trench from a side to a position beneath an elongated member.
Other objects of the invention include providing a trenching apparatus which can be operated by either remote control or by an operator on board the apparatus, which provides reliable and safe access to the operating control module or chamber, and which is particularly useful for trenching beneath a prelaid pipeline.